An internal combustion engine is provided with a number of cylinders, each of which is connected to an intake manifold by means of at least one intake valve and to an exhaust manifold by means of at least one exhaust valve. The intake manifold receives fresh air (i.e. air from the external environment) through a feeding pipe regulated by a butterfly valve and is connected to the cylinders by means of corresponding intake pipes, each of which is regulated by at least one intake valve.
The introduction of a swirl system, which is adapted to vary the section of the intake pipes during engine operation according to the speed of the engine itself (i.e. to the angular revolution speed of the crankshaft), has recently been suggested. At low speeds, the air introduction section through the intake pipes is decreased so as to generate turbulences in the intake air flow which improve the mixing of air and fuel in the cylinders; in virtue of the presence of these turbulences which improve mixing, all the injected fuel is burnt and thus the polluting emissions generated by the combustion are reduced. At high speeds, the introduction section of the air through the intake pipes is maximised so as to allow a complete filling of the cylinders and, thus, to allow the generation of the maximum possible power.
In order to vary the air introduction section through the intake pipes, each intake pipe presents two reciprocally parallel channels, only one of which may be fully closed by a butterfly choke valve. At low speeds, the butterfly choke valves are closed consequently reducing the air introduction section through the intake pipes, while at high speeds the butterfly choke valves are opened to maximise the air introduction section through the intake pipes.
Each butterfly valve comprises a butterfly valve plate fitted onto a shaft, which is rotationally mounted within the intake channel to turn about a rotation axis under the bias of an actuating device. Normally, the butterfly valve plate is welded onto the shaft, e.g. by spot laser welding. If the welding between the shaft and the butterfly valve plate is not perfectly performed, there is the risk that such welding may break in time, mainly due to the high pressures which occur within the intake channel at high speeds. In case of a fracture of the welding between shaft and butterfly valve plate, the butterfly valve plate itself may fall along the intake channel until it reaches the intake valves and thus damage the intake valves themselves.
Consequently, a rigorous inspection of each welding between a shaft and a butterfly valve plate is needed; however, such inspections considerably increase production costs of the butterfly valves of a swirl system.